Other research shows how placebo calories can have real effects on our metabolism and digestion. Participants in one study were given a 380-calorie chocolate milkshake, but they were told it was either a 620-calorie “indulgent shake” or a 140-calorie “sensi-shake.” The researchers took blood samples from the participants before and after they drank their milkshakes to measure ghrelin, a peptide released by the stomach that makes its way to the brain with this message: “Feed me!” The brain responds by generating the familiar sensation of hunger. As you eat, the stomach slows its release...